Laser diodes are typically used to transmit information over fiber optic networks. The laser diode may operate in a burst mode to achieve faster response times. In burst mode, the laser diode can be biased just around its radiation threshold in a standby or very low output power state. In this state the device outputs a minimum amount of optical energy into the surrounding network. When called upon to send information over the network the output power of the device must be rapidly ramped up to a level sufficient to transmit the data over the link length. This is done by quickly increasing the bias level current of the diode. The information being used to modulate the device is imparted upon the light by directly modulating the bias current with a signal representing the information. When the transmission is complete the bias current is once again reduced to a standby level and the output optical power, in-turn, is also reduced to a very low power standby level.
A burst mode laser is more efficient in terms of energy use and power dissipation. In contrast to a continuous mode laser, a burst mode laser transmitter produces an optical output only during selected intervals. It will be appreciated that the burst-mode transmitter is nearly turned off while it is in a standby stage and does not transmit an optical signal until a burst-mode incoming signal is received. Only upon receiving the incoming signal will the burst-mode transmitter operate, in comparison to the constant transmission of optical signals at the output of continuous mode laser transmitters. This manner of biasing a burst mode laser is illustrated in FIG. 1, which shows the optical power generated by the burst mode laser as a function of its bias current. The burst mode information, which may be encoded as a QAM (Quadrature Amplitude Modulation) signal, is also shown as being modulated on the optical output power.
It will be appreciated that the incoming signals used to bias the burst mode laser can be of various lengths of time, where some data bursts can be as short as a couple hundred nanoseconds, for instance, in the case of a DOCSIS burst signal. In a Gigabit PON (GPON) network, the minimum burst time is 32 ns including the preamble, delimiter and data. The minimum amount of data per burst is 1 byte (6.4 ns).
As indicated in FIG. 1, conventional burst mode optical transmitters that transmit analog or QAM encoded information cannot be completely turned off (no light radiation) during standby periods between bursts while also achieving the very rapid response time that is required of such systems. As a result, systems that employ these lasers must deal with the deleterious effects of the increased noise floor levels that arise when multiple laser transmitters in their lower power output state emit additive low level incoherent light into a common optical fiber transmission path. The increased noise floor levels can seriously affect the system's ability to reliably transmit analog information, including but not limited to QAM signals.